The thermal degree of efficiency of an internal combustion engine, in particular of spark ignition engines, is dependent on the compression ratio ε, i.e. the ratio of the total volume prior to compression to the compression volume (ε=(displacement volume Vh+compression volume Vc)/compression volume Vc). As the compression ratio increases, the thermal efficiency increases. The increase in the thermal efficiency over the compression ratio is degressive, but still relatively pronounced in the range of values that are typical nowadays.
In practice, the compression ratio can not be increased arbitrarily since an excessively high compression ratio leads to unintended self-ignition of the combustion mixture due to pressure and temperature increase. This early combustion does not only lead to spark ignition engines not running smoothly and so-called knocking, but can also lead to component damage in the engine. In the partial load range, the risk of self-ignition is less, which, in addition to the influence of ambient temperature and the pressure, also depends on the operating point of the engine. Accordingly, a higher compression ratio is possible in the partial load range. Efforts to match the compression ratio to the respective operating point of the engine have therefore been made in the development of modern combustion engines.
Various solutions exist for the realization of a variable compression ratio (VCR) with which the position of the crankshaft journal of the crankshaft or the piston pin of the engine piston is varied or the effective length of the connecting rod is varied. There are respective solutions for continuous and discontinuous adjustment of the components. Continuous adjustment makes it possible to reduce CO2 emissions and fuel consumption due to a compression ratio which can be adjusted for every operating point. On the other hand, discontinuous adjustment with two steps designed as end stops of the adjustment motion allows for structural and operational advantages and still allows for significant savings in fuel consumption and CO2 emissions compared to a conventional crankshaft drive.
U.S. Pat. No. 2,217,721 already describes an internal combustion engine with a length-adjustable connecting rod with two rod members which can be telescoped into one another and together form a high-pressure space. For filling the high-pressure space with and emptying it of engine oil and thus for changing the length of the connecting rod, a hydraulic adjustment mechanism is provided with a control valve having a spring-biased locking member which can be displaced to an open position due to the pressure of the engine oil.
Discontinuous adjustment of the compression ratio for an internal combustion engine is shown in EP 1 426 584 A1 in which an eccentric connected to the piston pin enables adjustment of the compression ratio. In this case, the eccentric is fixed at the one or the other end position of the pivoting range by use of a mechanical locking mechanism. DE 10 2005 055 199 A1 also discloses the mode of operation of a variable length connecting rod with which different compression ratios are enabled. There as well, the implementation is done by way of an eccentric in the connecting rod small end, which is fixed in position by two hydraulic cylinders with variable resistance.
WO 2013/092364 A1 describes a length-adjustable connecting rod for an internal combustion engine with two rod members that are telescopically displaceable into each other, one rod member forming a cylinder and the second rod member forming a piston element displaceable in length. A high-pressure space is formed between the adjustable piston of the first rod member and the cylinder of the second rod member, which is supplied with engine oil via a hydraulic adjustment mechanism with an oil channel and an oil-pressure-dependent valve. A similar length-adjustable connecting rod for an internal combustion engine with telescopically displaceable rod members is shown in WO 2015/055582 A2.
According to WO 2015/055582 A2, the compression ratio in the internal combustion engine is to be adjusted by way of the connecting rod length. The connecting rod length influences the compression volume in the combustion chamber, where the displacement volume is defined by the position of the crankshaft journal and the cylinder bore. A short connecting rod therefore leads to a smaller compression ratio than a long connecting rod with otherwise identical geometric dimensions, e.g. piston, cylinder head, crankshaft, valve timing, etc With the known length-adjustable connecting rods, the connecting rod length is varied hydraulically between two positions. The entire connecting rod is configured in several parts, where the change in length is effected by way of a telescopic mechanism which can be adjusted by use of a two-way hydraulic cylinder. The connecting rod small end, typically for receiving the piston pin, is connected to a piston rod (telescopic rod member). The associated adjustable piston is guided in an axially displaceable manner in a cylinder which is arranged in the connecting rod member with the connecting rod large end, typically for receiving the crankshaft journal. The adjustable piston separates the cylinder into two pressure spaces, an upper and a lower pressure space. These two pressure spaces are supplied with engine oil via a hydraulic adjustment mechanism, where the latter is supplied with engine oil from the lubrication of the connecting rod bearing. For this purpose, an oil channel is required from the crankshaft journal via the connecting rod bearing to the connecting rod and there via the check valves of the adjustment mechanism into the pressure spaces.
When the connecting rod is in the long position, there is no engine oil in the upper pressure space. The lower pressure space, however, is completely filled with engine oil. During operation, the connecting rod is subjected to alternating pull and push forces due to the gas and mass forces. In the long position of the connecting rod, a pull force is absorbed by mechanical contact with an upper stop of the adjustable piston. As a result, the connecting rod length does not change. A push force applied is transmitted via the piston surface to the lower pressure space filled with oil. Since the check valve of this space prevents oil return, the oil pressure rises, where very high dynamic pressures of well over 1,000 bar can occur in the lower pressure space. The connecting rod length does not change. The connecting rod is hydraulically locked in this direction by the system pressure.
In the short position of the connecting rod, the situation is reversed. The lower pressure space is empty, the upper pressure space is filled with engine oil. A pull force causes a pressure increase in the upper pressure space. A push force is absorbed by a mechanical stop.
The connecting rod length can be adjusted in two steps in that one of the two pressure spaces is emptied. For this purpose, one of the respective two check valves in the feed is bridged by the adjustment mechanism or an associated return flow channel is opened. Engine oil can flow through these return flow channels into the crankcase independently of the pressure difference between the pressure space and the supply device. The respective check valve loses its effect accordingly. The two return flow channels are opened and closed by a control valve, where precisely one return flow channel is always open, and the other is closed. The actuator for switching the two return flow channels is controlled hydraulically by the supply pressure.
The installation space for such a connecting rod is limited both axially and radially. The installation space in the crankshaft direction is limited by the bearing width and the spacing of the counterweights. In the axial direction, only the installation space between the connecting rod small end for supporting the piston pin and the bearing large end for supporting the crankshaft journal and a possible adjusting stroke of the connecting rod is available.
The forces to be transmitted by a connecting rod in an internal combustion engine are considerable, which is why the pressures in the pressure space of the cylinder-piston assembly can also be considerable. In view of the high internal pressures in such a cylinder-piston assembly and an associated hydraulic adjustment mechanism, the fatigue strength of the materials used is critical, but also the configuration of the components with regard to the small installation space.
A further aspect of a length-adjustable connecting rod with a cylinder-piston assembly for use in an internal combustion engine is that the hydraulic adjustment mechanism is typically supplied by the engine oil of the internal combustion engine, the viscosity of which decreases not only with the operating temperature but also with increasing operating time, thereby introducing harmful particles into the adjustment mechanism and the cylinder-piston assembly of the connecting rod. In addition to soot particles which can be generated during combustion in the engine, the engine oil also transports residual cast particles or swarf from the production and machining of the engine. Irrespective of a viscosity decrease of the engine oil and the particles transported into the adjustment mechanism by the engine oil, the adjustment mechanism of a length-adjustable connecting rod must remain operational for a long time.
With regard to the extreme pressure differences in the pressure spaces of a cylinder-piston assembly for a length-adjustable connecting rod of well over 1,000 bar and the effect of the force transmission via the connecting rod to the crankshaft on the power of the internal combustion engine, high-quality contacting sealing devices or structurally formed seals are used in conventional length-adjustable connecting rods. Any leakage from the respectively blocked pressure space leads to the adjustable piston entering into the respective pressure space, whereby a working amount corresponding to the force on the adjustable piston and the travel of the adjustable piston is dissipated, which leads to power loss of the internal combustion engine. Depending on the respective designs of the cylinder-piston assemblies, this power loss is to be deducted from the improved thermal efficiency of the internal combustion engine due to a variable compression ratio. Simple gap seals or piston seals are used as sealing devices in conventional length-adjustable connecting rods with a cylinder-piston assembly. Whereas gap seals have a certain leakage as a result of their design, piston seals as contacting sealing devices can almost prevent leakage. The advantages of gap seals are simple assembly, due to the smaller number of components, and a smaller installation space for the cylinder-piston assembly. On the other hand, the leakage of gap seals, which is inherent to the system, causes not only a power loss but also heat development in the system. in addition to increased aging of the engine oil, high temperatures in the length-adjustable connecting rod could lead to damage to the hydraulic adjustment mechanism and to problems with other components of the length-adjustable connecting rod due to thermal expansion.
Although piston-stroke engines are well-known in many fields of technology, and reciprocating piston engines are constantly optimized, improved and further developed in the automotive industry, the hydraulic adjustment mechanisms of cylinder-piston assemblies of the length-adjustable connecting rods continue to be unsatisfactory despite extensive development and research work, in particular, in terms of the necessary service life of length-adjustable connecting rods over the entire operating time of combustion engines. In conventional reciprocating piston engines, the hydraulic adjustment mechanisms of a cylinder-piston assembly of length-adjustable connecting rods, in addition to wear due to the metallic contact, are subjected to an increased load due to the small installation space available, the extreme temperature stress due to extremely high pressures, and the changing directions of force and also due to the contamination of the engine oil with soot particles and swarf. This leads to rapid wear of the sealing device and to the formation of grooves in the walls of the cylinder-piston assembly and ultimately to failure of the cylinder-piston assembly and power loss of the internal combustion engine.